An image display apparatus based on a hold-type display apparatus such as a liquid crystal display apparatus heretofore has had a problem in that quality of a moving image deteriorates (i.e., edge blur occurs).
The following explains deterioration of quality of a moving image (edge blur) of a conventional hold-type display apparatus, by taking, as an example, a case where as illustrated in FIG. 2, a region indicated by image signals having a luminance level of 75% horizontally moves on a background indicated by image signals having a luminance level of 25%.
FIG. 13 shows a distribution of luminance levels of input image signals which are supplied to pixels on one horizontal line within one screen image indicated by input image signals which are supplied in one frame in a case where an image illustrated in FIG. 2 is displayed.
FIG. 14 shows a temporal transition of a distribution of display luminances, for a case where the region horizontally moves on a screen of the conventional hold-type display apparatus. In general, a viewer watching a screen follows, with his eyes, an object which moves horizontally on the screen. Accordingly, the viewer recognizes, as a visual luminance level, an integral quantity obtained by integrating display luminance levels in a direction indicated by arrows.
The integral quantity, which is recognized by the viewer as the visual luminance level, is an average of a summation, in the direction of the arrows, of respective luminance levels of time periods which average is taken in a case where the object (i.e., area having a luminance level of 75%) moves at a speed of 8 pixels/frame. As shown in the graph of FIG. 14, the distribution of luminance levels is such that shaded regions having a luminance level of 75% are connected by inclined line segments in the vicinity of boundaries between regions having a luminance level of 25% and the shaded regions having a luminance level of 75%. A viewer recognizes, as edge blurs, (i) an area defined by a horizontal width between a rightmost inclined line segment and an adjacent one and (ii) an area defined by a horizontal width between a leftmost inclined line segment and an adjacent one. This causes deterioration of quality of a moving image of the hold-type image display apparatus.
The simplest methods for alleviating the edge blur include a method in which a minimum luminance level (i.e., black) display period is provided as a part of one frame period. According to the method, however, a bright screen image and a dark screen image are displayed in every one frame period. This causes a flicker. Every frame period has the minimum luminance level display period also in a case where an input image signal has a maximum signal level. This causes another problem of a decrease in luminance level.
The following describes a case where as illustrated in FIG. 4, a width of a region having a luminance level of 75% is smaller than a moving distance that the region moves, in one frame period, on a background having a luminance level of 25%.
FIG. 15 shows a distribution of luminance levels of input image signals which are supplied to pixels on one horizontal line within one screen image indicated by input image signals which are supplied in one frame period in a case where an image illustrated in FIG. 4 is displayed. FIG. 16 shows a temporal transition of a distribution of display luminances, for a case where the region horizontally moves on the screen of the conventional hold-type display apparatus.
The integral quantity, which is recognized by the viewer as the visual luminance level, is an average of a summation of respective luminance levels of time periods which average is taken in a case where an object (i.e., area having a luminance level of 75%) moves at a speed of 8 pixels/frame.
As illustrated in FIG. 17, a large edge blur such as the aforementioned one does not occur. On the other hand, a luminance level of the object which moves horizontally is considerably decreased from a normal luminance level of 75% to 44%. That is, the object looks considerably darker than a normal case. This also contributes to deterioration of quality of a moving image.
Conversely, in a case where the background has a high luminance level and the object has a low luminance level, there occurs another phenomenon such that the object looks brighter for a similar reason. This also contributes to deterioration of quality of a moving image.
Patent Literature 1 discloses a method for alleviating an edge blur without causing a flicker. According to the method, as illustrated in FIG. 17, a virtual frame image which is temporally located between two adjacent frames is generated by estimation so as to be inserted between the two adjacent frames. The method thus makes it possible to alleviate the edge blur so as to suppress deterioration of quality of a moving image.
However, the method has a difficulty in completely accurate estimation of image signals which are temporally located between the two adjacent frames. As a result, an error can be caused by inaccurate estimation.
With regard to one horizontal line within a screen on which as illustrated in FIG. 2, the object having a luminance level of 75% horizontally moves on the background having a luminance level of 25%, (a) of FIG. 18 shows an example of a luminance level distribution of input image signals of an (N−1)th frame, and (b) of FIG. 18 shows an example of a luminance level distribution of input image signals of an N-th frame. In a case where a virtual frame which is temporally located between the (N−1)th frame and the N-th frame can be generated in this case by accurate estimation, a luminance level distribution is such that as shown in (c) of FIG. 18, the region having a luminance level of 75% is located midway between the (N−1)th frame and the N-th frame. However, there is a difficulty in completely accurate estimation of image signals which are temporally located between two adjacent frames. As a result, an error can be caused by inaccurate estimation. (d) of FIG. 18 shows an example of a virtual frame containing such an error. As indicated by an arrow, a pixel having a luminance of 25% is located in a position where a luminance level is normally 75%.
On the other hand, Patent Literature 2 discloses a technique in which an image whose high spatial frequency has been removed and an image whose high spatial frequency is emphasized are repeatedly displayed so that a blur due to image movement is prevented. According to Patent Literature 2, however, both images are generated from one same input image. Accordingly, a deviation is caused in a relationship between space time and a luminance centroid with respect to a viewer who follows an object moving on a screen. This leads to a problem in that it is impossible to appropriately maintain a distribution of integrated luminances which are observed at a front edge and a rear edge of the displayed object. In addition, since positive values of high spatial frequency component are removed, an emphasized frame always has a high luminance. This leads to a problem in that a flicker occurs on an entire screen.
The problems of Patent Literatures 1 and 2 can be solved by a technique disclosed in the following Patent Literature 3 by inventors of the present invention.
According to Patent Literature 3, one frame period is divided into a sub-frame period A and a sub-frame period B, and a blurring process and an emphasizing process are carried out in the sub-frame period A and in the sub-frame period B, respectively. The technique thus makes it possible to improve quality of a moving image so as to solve the problem of Patent Literature 1.
Further, Patent Literature 3 discloses a technique in which a blurred frame is generated, and an average value between a latest frame and a previous frame is used. Thus, according to Patent Literature 3, it is possible to realize an appropriate relationship between space time and a luminance centroid with respect to a viewer who follows an object moving on a screen, so that it is possible to keep an appropriate distribution of integrated luminances which are observed at a front edge and at a rear edge of a displayed object. Further, according to Patent Literature 3, a broad luminance of the blurred frame and a broad luminance of the emphasizing frame are identical to each other, so that no flicker occurs on an entire screen.
Other than Patent Literature 3, techniques for suppressing a flicker include Patent Literatures 4, 5, and 6.